Amplitude limiter



March 6, 1951 E, W- HEROLD 2,544,226

AMPLITUDE LIMITER Filed Dec. 5, 1944 2 Sheets-Sheet 1 S Tlq'l E@f-NbgMiM/IL A B yJ mlm- L f i.

FPi

on u no ou no ooooooco encon nooo 0000 ccoo ooo oooao ooo enano ATTO R NEY 1 E. w. HEoLD AMPLITUDE LIMITER March 6, 1951l 2 Sheets-Sheet 2 FiledDeC. 5, 1944 Tlcn.

75 @cr/Ffa? .lTl mz U 3 v i i INVENTOR Y E N R O T T A Patented Mar. 6,1951 AMPLITUDE LIMITEE Edward W. Herold, Kingston, N. J., assigner toRadio Corporation of America, a corporation of Delaware ApplicationDecember 5, 1944, Serial No. 566,747

11 Claims. l

My present invention relates generally to amplitude limiters, and moreparticularly to a novel method of, and means for, limiting the amplitudeof high frequency signal waves.

In radio frequency communication systems it is frequently desirable toincorporate a device, commonly known as a limiter, which limits theamplitude of the receiver output signal to a xed value independent ofthe receiver input signal amplitude variation. For example, inl thereception of frequency modulated (FM) carrier waves it is found that asubstantial reduction in undesired interference and noise is achieved ifall amplitude variations can be eliminated before FM detection. This istrue, because while the intelligence is transmitted by carrier frequencyvariations independently oi` amplitude the undesired interference andnoise are present chiefly as variations in amplitude of the carrier.

One method of maintaining a radio receiver system with an outputintensity substantially independent of the input amplitude consists inthe application of automatic volume control (AVC) voltage to an amplier,as is commonly done in radio receivers. An AVC circuit, however, isoperative only under steady-state conditions,A because of the time delayinherent in the control circuit. It is not desirable to reduce the timedelay indefinitely. because uncontrollable oscillation would result. Asa result, rapid amplitude variations of the carrier are transmittedwithout reduction, and such controlled systems may be used for thereception of amplitude modulated carrier waves. For this reason, it isnot possible to achieve appreciable noise reduction by the use ofautomatic volume control in a frequency modulation receiver.

Still another method of obtaining amplitude limiting action consists inthe use of the voltage drop across a resistor to limit an electrodepotential applied to a vacuum tube, as, for example, by the use of ahigh series resistor in the signal input grid circuit of a tube.Limiting obtained by such means is not feasible at higher frequencies(such as those normally used in radio receivers), because of unavoidablecapacitive reactances. In the example of the series grid resistance, adegree of limiting is possible at higher frequencies, however, byshunting the resistance with a small capacitance. Again, although goodsteady-state performance is achieved, rapid carrier amplitudeiluctuations are not completely eliminated. The reaction time of thegrid resistance and its associated capacitances furthermore, is notusually the same for a sudden drop in amplitude as it is for a suddenrise in amplitude. The instantaneous limiting action is notsatisfactory, although a moderate degree of noise reduction is possiblewith this method in a frequency modulation receiver. Most o f the formsof amplitude limiter which have been proposed in the past are subject tolimitations similar to those enumerated above.

` An important object of my present invention is to provide anarrangement including an electron discharge device which achievesamplitude limiting, and which is, in most practical cases, instantaneousin action and free from the aforementioned objections of other methodsof limiting.

It is Well known that the maximum current which may be passed through ahigh-vacuum electron discharge device Vis inherently limited by spacecharge to a definite value which depends on electrode potentials. In myinvention such inherent limitation of current is used in a novel manner.for thepurpose of limiting the maximum output of a vacuum tube.

An important obiect of my invention is to provide an electron dischargedevice for limiting the output amplitude of substantially sinusoidalinput signals; said device having input terminals for the application ofsaid signals and output terminals whose current variations are used forobtaining the said amplitude limited output; the characteristic of thedevice being such that the fundamental component of the output currentwave reaches its final limiting value for input signal amplitudes lessthan required for the production of substantially square Waves of outputcurrent.

Another obiect of my invention is to provide an electron dischargedevice for amplitude limiting having an input electrode and an outputelectrode, and having such a characteristic when a sinusoidal inputvoltage is applied that the fundamental component of the outputelectrode current is substantiallv constant over all values of inputvoltage including those less than required for the production ofsubstantially square waves of output current.

Another object of my invention is to provide a method of, and means for,automatically controlling the shape of a limiting characteristic inresponse to variations in average signal level thereby to providesubstantially perfect instantaneous limiting action.

In the drawings:

Fig. l shows a limiter characteristic which is ideal, and should beapproached as closely as possible;

Fig. 2a shows two practicable forms of limiter characteristic, either ofwhich is attained by the invention;

Fig. 2b shows the input vs. output character' istics of a limitercorresponding to the two types of characteristics shown in Fig. 2a;

Fig. 3 shows a cross-sectional view, partly schematic, of one form oftube constructed according to the invention;

.age Aas possible.

Figs. 4a, 4b and 4c show respectively different characteristics typicalof a tube made as in Fig. 3;

Fig. 5 shows a part of an FM receiver embodying the invention, the tubeof Fig. 3 being schematically shown;

Fig. 6 is a circuit diagram of a modified amplitude limiter tubeemploying cascaded limit- 111g;

Fig. 7 schematically shows a modified form of the amplitude limitertube;

Fig. 8 shows a form of intercepting shield which can be employed in thetube of Fig. 7;

Fig. 9 shows a modification of the interception shield; and

Fig. 10 shows schematically the limiting characteristic secured whenutilizing a limiter tube employing the shield of Fig. 9.

Referring now to the accompanying drawing, wherein like referencecharacters in the different figures designates similar circuit elements,it will be obvious that an electron discharge tube which has acharacteristic curve of output current (Ib) vs. input electrode voltage(Ec) similar to curve A of Fig. 1 will function as an ideal amplitudelimiter when operated at the point indicated. As shown in Fig. 1, theapplication of a sinusoidal input voltage 'Ei of any amplitude whateverwill result in an output current I0 whose wave shape will be similar tothe curve B of Fig. l, i. e., a square wave. If a selective circuittransmitting only the fundamental frequency (and frequencies near it) isinserted in the plate circuit of such .a tube, only the amplitude of thefundamental component of current is effective in establishing an outputvoltage. With such a square wave of current, this fundamental currentcomponent will have a maximum value about 27% greater than the totalcurrent, and is represented by the dotted line curve C in Fig. 1. Sincethe total current is independent of the magnitude of the inputalternating voltage, the amplitude of iany component is also constantand perfect limiting is achieved. It should be noted that intelligencerepresented by frequency changes in the input wave is still present inthe output current.

In a practical tube lit is not usually possible to attain the ideal andperfect .characteristic represented by curve A of Fig. l. However, anapproach to an ideal characteristic may be secured, and usually theresulting characteristic is represented by curve D of Fig. 2a. Ananalysis shows that a tube with an Ip vs. Ec characteristic similar tocurve D of Fig. 2a gives a fundamental component of output current whichapproaches independence of input voltage at very large amplitudes ofinput, as would be expected. For smaller values of input voltage thefundamental component of current decreases, until at very small inputsthe tube no longer acts as a limiter. In a limiter, it is advantageousto have a condition of nearly perfect limiting at as small aninput volt-This may be achieved by a characteristic such as is ideally representedby the dotted line curve E of Fig. 2a which differs from curve B in thatas Ec is increased, the kcurve rises to a maximum, then slopes downward,and finally flattens out. The fundamental output current of such a tubeis y greater at small voltage inputs than for a tube with acharacteristic similar to curve D. At large voltage inputs thefundamental output from both tubes is the same. The voltage input vs.output current curve has, therefore, more Vnearly zero slope with a tubehaving the vcharacteristic `E than with one having characteristic D.This is shown in Fig. 2b where the input vs. output curves resultingfrom the two characteristics D and E are shown. In Fig. 2b there isplotted Alternating Input Voltage against Fundamental Frequency Current.The solid line curve corresponds to characteristic D of Fig. 2a, Whilethe dotted line curve corresponds to characteristic E of Fig. 2a. Thetube of my present invention can be made to have a characteristicresembling either of curves D or E of Fig. 2a, when properly operated.This advantage is not shared by many `previously known forms of limiterwhich have characteristics that approach characteristic D only.

A vertical cross-sectional View of one embodiment of an electrondischarge tube suitable for use in my invention is shown in Fig. 3. Thetube envelope I in Fig. 3 may be of glass or metal. It is shown brokenaway at the lower end thereof. The electrodes of the tubes `areschematically represented. Those skilled in the art of constructingelectron discharge tubes will be fully cognizant of the manner ofmanufacturing the tube to be described. It is to be understood that thetube of Fig. 3 is only an example of the invention. The cold electrodesare all concentri- :cally located relative to the vertical cathode 2.The cathode 2 is surrounded by four concentric, foraminous, coldelectrodes and an output plate or anode.

The first electrode 3 surrounding the cathode 2 is a control, or signal,electrode on which is impressed the input vsignal whose amplitude is tobe limited. A suitable negative bias voltage is, also, shown applied toelectrode 3. The next adjacent electrode 4 is an accelerating electrodewhich is operated at a constant positive potential with respect to thecathode. The electrode 4 serves to establish a sufficiently positivefield to draw electrons from the cathode when the potential or electrode3 is not too negative. Electrode 5 is a retarding electrode which isconnected to a constant source of potential having a value not greatlydifferent from that of the cathode, and preferably slightly negativewith respect to it. Electrode ii is a second accelerating electrode, andis operated at a constant positive potential with respect to thecathode. The output anode or plate 'l is adapted to be connected to asignal output circuit, and has applied to it a constant positive sourceof potential which is preferably higher than that applied to electrode Gin order to avoid the disturbing effects of secondary electron emission.Alternatively, of course, an additional suppressor electrode (not shown)could bev used for the reduction of secondary emission effects.

The amount of current leaving the cathode of the tube of Fig. 3 isdetermined by the instantaneous value of the potential of electrode 3which is Varied by the incoming signal. This current increases withincreasing potential on electrode 3. By increase is meant in a positivepotential sense. The electrons comprising the current pass throughelectrode 4 and enter the retarding field between electrodes i and 5,where they are slowed down. The space charge resulting from large'numbers of electrons at 10W velocities serves considerably' to lowerthe space potential between electrodes 4 and 5. As is well known, if asufiicient number of electrons (i. e., a sufficiently high current) ispresent, the space potential may fall to that of the cathode potentialand many electrons will be slowed down to zero forwar velocity andturned about. The point at which the space potential is lowered to thatof the cathode potential is called a virtual cathode, and thisphenomenon has frequently been studied experimentally and theoretically.As is also Well known, a further increase of current, as caused by anincrease in potential of electrode 3 beyond that necessary to produce avirtual cathode, results in a shift in position of this virtual cathodein a direction so as to permit less current to reach the anode 'I. It isthus clear that with a uniformly increasing potential in a positivesense on electrode 3 the anode current will first rise, then reach amaximum, and decrease again. It will be noted, however, that electrode 5has non-uniform openings. That is, the space 3 is provided along theturns.

Hence, the current which will produce a virtual cathode at the endportion of the space between electrodes 4 and 5 is not yet suiiicient toproduce such a condition at the center portion in alignment with therelatively large space 8. As a result, the decrease in anode current isnot as marked as would be found if electrode 5 were con-- structeduniform, and, under some conditions, only a small decrease in anodecurrent may be obtained with a large increase in cathode current. Thenon-uniform section'electrode could, of course, be provided in any othermanner, if desired. Indeed, space 3 could be provided at any point alongthe length of electrode 5.

Curves of output electrode current vs. input electrode voltage on tubesmade similarly to that of Fig. 3 are shown in Figs. 4a, 4b and 4c.respective It vs. Ec curves were obtained with different operatingconditions on the same tube, and show the wide variety of usefulcharacteristics which it is possible to obtain. Each of the threecharacteristics F, G and H of respective Figs. 4a, 4b and llc has valuein amplitude limiting, although as already mentioned curve G is probablythe best for general use since it closely approaches curve E of Fig. 2a.

The utility of the curve E of Fig. 2a, in an FM receiver is that, when asignal just too small to be adequately limited by curve D is received,curve` E permits by virtue of its hump more output. Thus, when theoutput from curve D is decreasing due to too small a signal, the outputof E remains high. In other words, the lower limit of signal level,suicient to cause good limiting, is less with curve E than with D. Tomake this still more clear, suppose a limiter of type D requires 2 voltsto swing it well up, so as to produce nearly constant output. Thereceiver designer puts in, say, a gain of 100,000 so -that the antennavoltage needed is 2/l00,000 or 20 uv. as a minimum. When the antennavoltage is less, say l av., the limiter voltage is only 1 volt and theoutput of the limiter is less than for 2 volts, i. e., the output varieswith input and is decreasing as the input decreases. This meansinadequate limiting. Curve E, however, can be made so that the 1 voltinput produces just as much output as the 2 volt, or even a l0 voltinput. Thus, if the receiver designer wishes, he need use a gain of only50,000 to handle a 20 av. signal. Alternatively he may retain hisamplifier of gain 100,000 and he will have adequate limiting for the l0liv. signal which did not work for curve D. Curve F of Fig. la isproduced by a more negative bias on grid 5 of the tube of Fig. 3. CurveH of Fig. 4c is produced with less negative bias on grid 5. It hasrelatively little utility at present, be-

They

tube.

cause of the top upwardly-sloping part of the curve. Curve G is probablyan optimum.

It has been found possible to control the shape of the limitingcharacteristic to some extent by adjustment of the potential of oneelectrode such as, for example, electrode 5 of Fig. 3. It is possible tomake use of this adjustment to control the Ib vs. Ec characteristicautomatically so as more nearly to approach perfect instantaneouslimiting even with wide variation of average signal level. This may bedone when the average signal input amplitude varies relatively slowlycompared to the instantaneous changes in amplitude, as with some typesof carrier fading. The

potential of the electrode controlling the limiting characteristic isthen varied automatically `in accordance with slow average carrieramplitude variations. For this purpose there may be used some form ofcontrol similar to that known as automatic volume control (AVC) inconventional radio broadcast receivers.

In Fig. 5 I have shown a receiving system of angle modulated carrierwaves embodying a limiter circuit constructed in accordance with theprinciples of my invention. The term angle modulated is generic, andincludes frequency modulation (FM), phase modulation (PM) or hybridmodulation possessing characteristics common to both FM and PM. For thesake of simplicity let it be assumed that the receiver is ol thesuperheterodyne type, is adapted to receive FM carrier waves in theLl0-50 megacycle (mc.) range, and that the maximum frequency swing iskilocycles (kc.) at the transmitter. Those skilled in the art of FMcommunication know that the FM transmitter carrier is deviated infrequency from its normal frequency to an extent dependent on themodulation signal amplitude, while the rate of deviation is dependent onthe modulation signal frequencies per se.

The permitted deviation in the present FM band of 40-50 mc. is m75 kc.The invention is not limited to the specific frequencies or rangesmentioned. The collected FM carrier waves are reduced in frequency tothe operating intermediate frequency (I. F.) value. The latter may be4.3 mc. by way of specific example. The networks prior to the limitertube I may be those which are suitable for FM reception in asuperheterodyne receiver. The selector circuits between tubes shouldpass the maximum frequency swing of the Waves, and may desirably havepass bands about 200 kc. wide. Thus, the numeral I0 denotes the I. F.band pass transformer whose primary circuit II and secondary circuit i2are each tuned to 4.3 mc. The pass band of network II, I2 is 200 kc. Itwill be understood that circuit II is arranged in the plate circuit ofthe last I. F. amplifier tube, or in the plate circuit of the converterThe limiter tube I has its electrodes corresponding to the sectionedtube shown in Fig. 3. The cathode 2 is grounded, and thesignal controlelectrode or grid 3 is connected to the high alternating potential sideof input circuit I2. The source I3 of negative biasing voltage is shownestablishing grid 3 at a suitable negative bias. The low potential sideof circuit I 2 is connected to ground for I. F. currents by condenserI4. Grids 4 and E are at positive potentials, and are suitably bypassedto ground for I. F. currents. Plate 1,

. which is at +B potential, is returned to ground for I. F. currents bycondenser I5.

The grid 5 provided with space 8 is located be- ,75 tween grids 4 and 6,and is connected by resistor the anode 2B of diode I9 is provided.Resistor I8 is connected in the diode space current path, the I. F.signal energy being applied to cathode 2| through condenser 23. Thelatter condenser has its input electrode connected to the grid side ofinput circuit I2.

The controlled grid 5 has its initial bias as provided by slider 22,and, in addition, is regulated in potential in accordance withvariations of voltage across resistor I8. The upper end of resistor I5is connected to the grounded cathode by condenser 24. The resistor I6and condenser 2li cooperate to provide a simple iilter network to removerapid iluctuations of voltage. The constants of the network I5, 24 areso chosen however, that the voltage of grid I5 will respond to anyrelatively slow changes in carrier amplitude occurring'at transformerI0, such as When a strong and then a Weak signal are tuned in. In otherwords, the diode I9 and its pertinent circuit elements act in the mannerof delayed AVC. The amount of delay bias applied to the diode I9, asgiven by the voltage difference between sliders 22 and 5I), determinesthe carrier amplitude level above which regulation takes place.

The relatively rapid variations in amplitude of the FM waves (whosecenter frequency is at I. F.) are substantially reduced by the tube Idue to the action described in connection with Fig. 3. The amplitudelimiting at the tube I is desirable, since the FM waves acquireconsiderable also responds over much of its characteristic to Y.amplitude modulation, per se.

Hence, any undesired amplitude modulation on the FM carrier prior todetection will show up in the discriminator output as a spurious andundesired com-l ponent.

Hence, it is desirable to use an amplitude limiter prior to thediscriminator thereby to eliminate the undesired amplitude modulationcomponents.v Only the discriminator section of the FM detector is shown,since the FM detector forms no part of the present invention. Thediscriminator may be of any suitable form Wellknown to those skilled inthe art. That specically shown is described and claimed by S. W. Seeleyin U. S. Patent No. 2,121,203, granted June 2l, 1938. It is suiiicientfor the purpose of this application to describe it as generallycomprising a transformer 25 whose primary and secondary tuned circuits25 and 21 are each tuned to the I. F. value, and the midpoint ofsecondary coil 28 is connected through blocking condenser 29 to theplate side of primary circuit 26. The opposite sides of secondarycircuit 21 are connected to respective input electrodes of a pair ofopposed rectiers, whose rectified youtput voltages are differentiallycombined to provide a modulation signal voltage corresponding solely -tothe frequency deviations of the FM carrier waves.

As explained previously, the virtual cathode effect produced between theelectrodes 4 and 5 is delayed or retarded at the space in the vicinityof opening 8. The characteristic of tube I will be similar to F orG'depending on the potential of grid 5. The diode I9 in Fig. 5 rectiflesthe I. F. input signal Whenever it exceeds the potential between tap 22and tap 50, so that a direct current potential is then created acrosspotentiometer resistor I8. An increase in the average value of the inputsignal above the point at which diode I9 conducts increases the directcurrent potential across resistor I8. By applying part of this potentialto the retarding electrode 5 through resistor I6, the potential ofelectrode 5 is automatically varied with slow variations in the averagevalue of the input signal. Rapid variations, of course, are not passedby the filter network I6, 24.

The polarity of the diode connection is such that an increase in averagesignal amplitude causes the potential of the grid 5 to become morepositive. This, in turn, changes the limiter characteristic from onewith a downward slope (as curve F of Fig. 4a) to a curve which is y'morenearly ilat (as curve G or H of Figs. 4b and 4c respectively). Such achange is in the right direction to improve instantaneous limiting atany one average signal amplitude. Initial bias voltages on the signalelectrode 3 and on the retarding electrode 5 are provided by currentsource I3 as here-ofore explained. In many instances, suiicient limitingis obtained without the use of diode I9. In this case, grid 5 may beconnected directly to slider 22 by adjustment of slider I1 -to itslowest `point and slider 22 is adjusted to give the characteristic G ofFig. 4b. In this case, the diode plays no part in the circuit and thetube characteristic closely resembles E of Fig. 2a.

It is frequently of advantage to obtain multiple limiting in the sameelectrode structure. This may be done by the use of two or moresuccessive virtual cathodes along the path of the same electron stream.It has, also, been found advantageous to combine different forms oflimiting n one tube, such as combined limitation due .to saturatedemission from a thermionic cathode or filament with virtual cathodelimitation. In Fig. 6 I have shown multiple limiting performed in tubeI', which generally is similar to the tube I shown in Fig. 5. The addedelectrodes are auxiliary retarding elec .rode 5 and additional positiveelectrode E. Electrodes Il, 6 and 6 are at a common positive potential,and retarding electrode 5', which may have an opening 8 similar toopening 8 of electrode 5, is at a more negative potential than grid 5.

The Atwo negative retarding grids 5 and 5 each have a virtual ca.hode ontheir side closest to the cathode by virtue of the spacings andpotentials applied, and each retarding grid is provided with an opening8 in the center thereof. Thus, the rst virtual cathode region limits thecurrent iiow through grid 6, after which a second limiting action aheadof grid 5' limits the current again which can ow to grid 6' and anode l.This is cascade limiting in an electronic sense of the term.

Another type of cascade limiting would involve the use of the tube shownin Fig. 3 at reduced iilament voltage. This rst limits the totalavailable current after which retarding grid 5 again 9, .Y limits theelectron current ow. In any of these arrangements it is desirable toinsure the uitirnate limiting characteristic to have the desired' humpor peak as exemplified by curve E ofv focussing electrodes make up theelectron gun,`

and for this` reason the latter is schematicallt7 represented as asource of electrons 3l followed by electrodes 3i, the electron gunemitting electrons in the form of a beam 32.

The deflection plates "33; betweenv which the rectangular beam 32 passesin normal symmetry, are connected lto respective sides of the inputcircuit I2. The midpoint of the input coil is shown connected to thepositive terminal of a direct current biasing source 34.

tron beam 32, and the beam is normally positioned midway between theplates 33. The anode 35 is connected to the high potentialvside ofoutput circuit 26, and the beam falls on theanode to provide currentflowthrough output circuit 26.

Control over the deiiection plates in 4responseto an input signal causesthe beam to travel overV the edge of intercepting electrode 3G. Thelatter is shown, in Fig. 7, located broadside to the beam 32. That is,in Fig. 7 Ythe end edge K-K of electrode 36 is seen, as indicated inFigs. 8 and 9.l

The electrode 36 is given a positive bias.

In Fig. 8 there is shown the configuration ofelectrode 35 as seen fromthe direction of the beam 32. The beam i cross-section is shown dotted,and its direction of travel is indicated by arrows. The shape of theelectrode 36 shown in Fig. 8 will produce the form of characteristicshown in curve G of Fig. 4b, the deflection bias being Ec in that case;The shape of electrode 36 shown in Fig. 9 will provide an fAnode Currentvs. Deflection Bias" characteristic as illustrated by curve ,J in Fig.l0, which behaves similarly to curves E and G of Figs. 2 and; 4 but issymmetrical. The tube of Fig. 7 may advantageously be made so that theanode 35 has a high secondary emission ratio by proper choice of anodematerial. In this event, the anode potential may be slightly less thanthatof the intercepting electrode 36, and a multiplication of the outputcurrent occurs due to the large number of secondary electrons whichleave the anode to be collected by electrode In a beam-deflectiontube,of the type I have shown in Fig. '7, an extremely large input signal mayswing the beam so far as to strike the walls of the tube or some "otherundesired tube part.

Ordinarily the limiter tube of my invention will be preceded by anamplifier whose overload point It is only necessary, in that case, toernf In Fig. 'l I have shown an electron beam tube 30 which may be ofthe type' gen' Hence, normally thek plates 33 have equal attractiveeffects on the elec-` 10 is such that this extreme signal condition isnot encountered. However, if this is not so, the con-v dition is easilyand simply avoided by means well known in the art wherein AVC orpreferably delayed AVC is applied to prior tubes in the amplier chain.As heretofore explained, the AVC action will not eect instantaneouslimiting, but it will protect the limiter 'tube from excessive inputsignals so that it may properly function as an instantaneous limiter,The addition of such an AVC system in no wise alters my basic in-Vvention and is not a part of it; thus it has not been shown in thefigures.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention as set forth in the appendedclaims.

What I claim is:

l. In a method of limiting the amplitude of output current resultingfrom sinusoidal input signals, the steps of producing a stream ofelectrons, controlling the electron stream in response to said signals,collecting said controlled stream to provide output current, producing avirtual cathode extending over a substantial portion only of the streambetween the controlling point and the collecting point so that thevirtual cathode is only partially effective to limit the output currentamplitude whereby the characteristic relating output current to inputsignal variation possesses a peak prior to a final limiting value whichis less than said peak value.

2. In a method of limiting the amplitude of output current resultingfrom sinusoidal input signals, the steps of producing a stream ofelectrons, controlling the electron stream in response controlling pointand the collecting point so that the virtual cathode is only partiallyeffective to limit the output current amplitude whereby thecharacteristic relating output current to input signal variationpossesses a peak prior to a nal limiting value which is less than saidpeak value, and automatically regulating the shape of saidcharacteristic by decreasing the number of electrons constituting thevirtual cathode in response to relatively slow increases in input signalamplitude.A-

3. In a limiter tube of the type provided with at least a cathode,control grid, acceleration grid, retarding grid and anode in the ordernamed, means for applying negative biases to the control grid andretarding grid, means for applying positive voltages to the accelerationgrid and anode, said retarding grid having openings in an intermediateportion of said retarding grid which are wider than the openings in theremaining portion of said grid thereby to provide a virtual cathode areabetween the acceleration grid and the retarding grid which is moreeffective over said remaining grid portion.

4. In a limiter'tube of the type provided with at least a cathode,control grid, acceleration grid, retarding grid and anode in the ordernamed, means for applying negative biases to the control grid andretarding grid, means for applying posin tive voltages to theacceleration grid and anode,

said retarding grid having openings in a center than the openings in theremaining portion of said grid to provide a virtual cathode area betweenthe acceleration grid and the retarding grid which is more effectiveover said remaining grid portion, and means providing between saidretarding grid and anode a second virtual cathode area which is moreeffective over a substantial predetermined portion of said area thanoverk the remaining portion thereof.

5. In a method of limiting the amplitude of output current resultingfrom sinusoidal input signals, the steps of producing a stream ofelectrons, controlling the electron stream in response to said signals,collecting said controlled stream to provide output current, producing avirtual cathode extending over a substantial portion only of thecross-section of the electron stream between the controlling point andthe collecting point, and automatically controlling the number ofelectrons constituting the virtual cathode by decreasing or increasingsaid number in response to relatively slow increases or decreases ininput signal amplitude.

6. An electron vdischarge device adapted for amplitude limiting andcomprising a cathode, a control grid, an anode and electrode meanspositioned between said control grid and said anode for providing avirtual cathode between said control grid and said anode, said electrodemeans including a retarding grid having a variably spaced grid windingto provide wider openi'ngs over a predetermined portion of saidretarding grid than over the remaining portion thereof, whereby asinusoidal input voltage impressed on said cathode and control grid willproduce an anode current having a fundamental component which issubstantially constant over all values of input voltage including thoseless than required for the production of substantially square waves ofanode output current. Y

7. An electron discharge device adapted for amplitude limiting andcomprising a cathode, a control grid, an anode and electrode meanspositioned between said control grid and said anode for providing avirtual cathode between said control grid and said anode, said electrodemeans including a retarding grid having a variable pitch grid winding toprovide wider openings over a predetermined portion of said retardinggrid than over the remaining portion thereof, and a voltage supply formaintaining said retarding grid at a voltage which is negative withrespect to that of said cathode, whereby a sinusoidal input voltageimpressed on said cathode and control grid will produce an anode currenthaving a fundamental component which is substantially constant over allvalues of input voltage including those less than required for theproduction of substantially square waves of anode output current,

8. An electron discharge device adapted for amplitude limiting andcomprising a cathode, a control grid, an anode and electrode meanspositioned between said control grid and said anode for providing avirtual cathode between said control grid and said anode, said electrodemeans including two accelerating grids and a retarding grid positionedbetween said accelerating grids, said retarding grid having openings ina center portion thereof which are wider than the openings in theremaining portion thereof, and a voltage source for supplying potentialsto said accelerating grids which are positive with respect 12" to thatof said cathode and for supplying a poten-'- tial to said retarding gridwhich is negative with respect to that of said cathode, whereby asinusoidal input voltage impressed on said cathode and control grid willproduce an anode current having a fundamental component which issubstantially constant over all values of input voltage including thoseless than required for the proy duction of substantially square waves ofanode l0` non-uniform control effect on the electron stream of saidtube, and a source of power for applying a bias voltage to saidretarding grid which is negative with respect to that of said cathode toprovide a limiter characteristic which rises to a peak and thereafterdecreases again.

10. In combination with a source of frequency modulated waves, anamplitude limiter comprising a v-acuum discharge tube having a cathode,a control grid, an anode and a retarding grid, said retarding gridhaving a variable pitch grid winding to provide wider openings over apredetermined portion of said retarding grid than over the remainingportion thereof and to exhibit a non-uniform control effect on theelectron stream of said tube, a voltage source for applying a biasvoltage to said retarding grid which is negative with respect to that ofsaid cathode to provide a limiter characteristic which rises to a peakand thereafter decreases again, and means responsive to relatively slowcarrier amplitude changes for rendering said bias voltage more positivewhen v said carrier -amplitude increases.

REFERENCES CITED rhe following references are of record in the i'lle ofthis patent:

UNITED STATES PATENTS Number Name Date 1,757,345 Strobel May 6, 19302,226,752 Eagleseld Dec. 31, 1940 2,262,406 Rath Nov. 11, 1941 2,306,457Mayne Dec, 29, 1942 2,324,090 Lange July 13, 1943 2,357,405 Katzin Mar.1l, 1944 2,395,615 Curtis Feb. 26, 1946 FOREIGN PATENTS Number CountryDate 559,218 Great Britain Feb. 9, 1944

